A. Technical Field
The present invention relates to: a vinylpyrrolidone polymer which is excellent in the storage stability; a composition containing the vinylpyrrolidone polymer; a stabilization process for the vinylpyrrolidone polymer; and a preservation process for the vinylpyrrolidone polymer.
B. Background Art
A vinylpyrrolidone polymer, such as poly(vinylpyrrolidone) or a vinylpyrrolidone copolymer, is widely used in various fields such as medicines, cosmetics, pressure sensitive adhesives or adhesives, paints, dispersants, inks, and electronic parts, because the vinylpyrrolidone polymer has merits or advantages of biocompatibility, safety, hydrophilicity, and so on.
However, while providing the vinylpyrrolidone polymer to the above various uses, the present applicant found that the vinylpyrrolidone polymer had problems in respect to the stability of storability and so on. For example, whether the vinylpyrrolidone polymer is a powder or solution, there were cases where the vinylpyirolidone polymer or a solution thereof got colored or underwent the reduction of the molecular weight or viscosity when stored for a long time or at high temperature.
A. Object of the Invention
An object of the present invention is to provide: a vinylpyrrolidone polymer which has extremely good storage stability and thermal stability in that even if the vinylpyrrolidone polymer is stored for a long time, coloring or other changes of physical properties occur little, and the thermal stability is also excellent, and the storage in a solution state is also possible; a composition containing the vinylpyrrolidone polymer; and a stabilization process for the vinylpyrrolidone polymer.
Another object of the present invention is to provide a preservation process for a vinylpyrrolidone polymer by which: even when the vinylpyrrolidone polymer is preserved for a long time or at high temperature, the physical properties such as molecular weight (K value) of the vinylpyrrolidone polymer can be prevented from changing, therefore the vinylpyrrolidone polymer can stably be preserved.
B. Disclosure of the Invention
The present inventor diligently studied to solve the above problems. As a result, the inventor found that the above problems could be solved if the vinylpyrrolidone polymer was mixed with a certain amount of antioxidant, and further that a process, comprising the step of examining how much a K value, as defined by Fikentscher""s equation, reduced when heating the vinylpyrrolidone polymer under prescribed conditions, was good as a process for exactly judging the storage stability of the vinylpyrrolidone polymer. The inventor also found that the above problems could be solved if the oxygen concentration in a gas phase that contacts with the vinylpyrrolidone polymer was kept to a certain amount or less when preserving. The present invention has been completed on the basis of these findings.
Therefore, a vinylpyrrolidone polymer, according to the present invention, contains at least one antioxidant in a ratio of 0.00001xcx9c30 weight %, preferably 0.001xcx9c30 weight %, to the vinylpyrrolidone polymer. Another vinylpyrrolidone polymer, according to the present invention, is adjusted such that the reduction ratio of the K value defined by Fikentscher""s equation will be within 5% when the vinylpyrrolidone polymer is subjected to a forcible test in which the vinylpyrrolidone polymer is kept heated at 120xc2x0 C. under normal pressure in air for 2 hours. Yet another vinylpyrrolidone polymer, according to the present invention, is adjusted such that the reduction ratio of the K value defined by Fikentscher""s equation will be within 5% when the vinylpyrrolidone polymer is subjected to a promotion test in which the vinylpyrrolidone polymer is kept heated at 50xc2x0 C. in a solution state for 14 days.
A composition for various uses, according to the present invention, comprises a vinylpyrrolidone polymer as a resin component, and further comprises at least one antioxidant in a ratio of 0.00001xcx9c30 weight %, preferably 0.001xcx9c30 weight %, to the vinylpyrrolidone polymer.
A stabilization process for a vinylpyrrolidone polymer, according to the present invention, comprises the step of mixing the vinylpyrrolidone polymer with at least one antioxidant in a ratio of 0.00001xcx9c30 weight %, preferably 0.001xcx9c30 weight %, to the vinylpyrrolidone polymer.
A preservation process for a vinylpyrrolidone polymer, according to the present invention, comprises the step of suppressing the oxygen concentration to not higher than 50,000 ppm in a gas phase that contacts with the vinylpyrrolidone polymer.
These and other objects and the advantages of the present invention will be more fully apparent from the following detailed disclosure.
Hereinafter, a detailed description is given about a mode for carrying out the present invention.
The vinylpyrrolidone polymer in the present invention is, specifically, poly(vinylpyrrolidone) and/or a vinylpyrrolidone copolymer.
The poly(vinylpyrrolidone) is a compound of general formula (3) below: 
(wherein n is an integer) and is obtained by polymerizing vinylpyrrolidone by any method as mentioned below.
The vinylpyrrolidone copolymer is a copolymer of which the constituents include vinylpyrrolidone and a comonomer copolymerizable therewith. The comonomer to be copolymerized with vinylpyrrolidone is not especially limited, but specific examples thereof include: 1) (meth)acrylate esters such as methyl(meth)acrylate, ethyl(meth)acrylate, butyl(meth)acrylate, cyclohexyl(meth)acrylate, and hydroxyethyl(meth)acrylate; 2) (meth)acrylamide and derivatives therefrom such as N-monomethyl(meth)acrylamide, N-monoethyl(meth)acrylamide, and N,N-dimethyl(meth)acrylamide; 3) basic unsaturated monomers such as dimethylaminoethyl(meth)acrylate, dimethylaminoethyl(meth)acrylamide, vinylpyridine, and vinylimidazole and their salts or quaternized products; 4) vinylamides such as vinylformamide, vinylacetamide, and vinyloxazolidone; 5) carboxyl-group-containing unsaturated monomers such as (meth)acrylic acid, itaconic acid, maleic acid, and fumaric acid; 6) unsaturated anhydrides such as maleic anhydride and itaconic anhydride; 7) vinyl esters such as vinyl acetate and vinyl propionate; 8) vinylethylene carbonate and derivatives therefrom; 9) styrene and derivatives therefrom; 10) 2-sulfoethyl(meth)acrylate and derivatives therefrom; 11) vinylsulfonic acid and derivatives therefrom; 12) vinyl ethers such as methyl vinyl ether, ethyl vinyl ether, and butyl vinyl ether; and 13) olefins such as ethylene, propylene, octene, and butadiene. Among these groups of comonomers, particularly, groups 1)xcx9c8) are favorable, for example, in respect to the copolymerizability with vinylpyrrolidone. As to the above comonomers, only one kind may be copolymerized with vinylpyrrolidone, or any mixture of two or more kinds of comonomers may be copolymerized with vinylpyrrolidone.
The ratio of vinylpyrrolidone to the entire monomer components of the vinylpyrrolidone copolymer is not especially limited, but is preferably not lower than 0.1 mol %, more preferably not lower than 1.0 mol %, still more preferably not lower than 5.0 mol %, most preferably not lower than 20.0 mol %. In the case where the ratio of vinylpyrrolidone in the copolymer is lower than 0.1 mol %, there are disadvantages in that various properties deriving from vinylpyrrolidone cannot be exhibited and the deterioration of the polymer to be solved by the present invention is seen little.
The polymerization reaction to give the vinylpyrrolidone polymer can be carried out by conventional polymerization methods such as bulk polymerization, solution polymerization, emulsion polymerization, suspension polymerization, and precipitation polymerization. The reaction temperature in the polymerization reaction may fitly be set according to conditions such as reactants, but is preferably in the range of 0xcx9c250xc2x0 C., more preferably 20xcx9c150xc2x0 C., most preferably 40xcx9c100xc2x0 C.
The polymerization reaction to give the vinylpyrrolidone polymer may be carried out by adding conventional polymerization initiators, of which the specific examples include: radical polymerization initiators such as azo compounds (e.g. 2,2xe2x80x2-azobisisobutyronitrile and 2,2xe2x80x2-azobis(2-methylpropionamidine)dihydrochloride) and peroxides (e.g. benzoyl peroxide and hydrogen peroxide); and cationic polymerization initiators such as boron trifluoride or complexes thereof, iron(II) chloride, diethylaluminum chloride, diethylzinc, heteropolyacids, and activated clay. The concentration of the polymerization initiator in the polymerization reaction is not especially limited, but is preferably in the range of 0.001xcx9c10 weight %, more preferably 0.01xcx9c5 weight %, most preferably 0.05xcx9c3 weight %, of the monomer components. When the polymerization reaction is carried out, for example, any chain transfer agent, pH-adjusting agent, or buffer agent may fitly be used besides the polymerization initiator as the need arises.
The solvent, as used for the polymerization reaction to give the vinylpyrrolidone polymer, or as used when preserving the vinylpyrrolidone polymer in a solution state, is not especially limited, but examples thereof include: water; alcohols such as methyl alcohol, ethyl alcohol, isopropyl alcohol, diethylene glycol; alkylene glycol ethers (acetates) such as propylene glycol monomethyl acetate and diethylene glycol monomethyl ether acetate; amides such as dimethylformamide and N-methylpyrrolidone; esters such as ethyl acetate, butyl acetate, and xcex3-butyrolactone; aliphatic hydrocarbons such as hexane and octane; alicyclic saturated hydrocarbons such as cyclohexane; alicyclic unsaturated hydrocarbons such as cyclohexene; aromatic hydrocarbons such as benzene, toluene, and xylene; ketones such as acetone and methyl ethyl ketone; halogenated hydrocarbons such as dichloroethane, chloroform, and carbon tetrachloride; ethers such as diethyl ether, dioxane, and tetrahydrofuran; sulfonate esters such as dimethyl sulfoxide; carbonate esters such as dimethyl carbonate and diethyl carbonate; and alicyclic carbonate esters such as ethylene carbonate and propylene carbonate. Among these, particularly, the ethers (acetates) and the amides are preferable, and water and the alcohols are more preferable. These solvents may be used alone respectively or in combinations with each other. In addition, the above solvents are preferably used such that the concentration of the monomer components in the raw material mixture in the polymerization reaction can be preferably in the range of 1xcx9c99 weight %, more preferably 5xcx9c70 weight %, most preferably 10xcx9c60 weight %.
In the present invention, the stabilization of the properties, such as storability and heat resistance, of the vinylpyrrolidone polymer is designed by mixing the above vinylpyrrolidone polymer with at least one antioxidant in a ratio of 0.00001xcx9c30 weight %, preferably 0.001xcx9c30 weight %, to the vinylpyrrolidone polymer. In the case where the mixing ratio of the antioxidant is lower than 0.00001 weight %, the stabilization of the properties such as storability and heat resistance is difficult. In the case where the ratio is higher than 30 weight %, there are problems in that the inherent properties, such as strength and appearance, of the polymer are damaged.
Examples of the antioxidant usable in the present invention include:
phenolic antioxidants such as sodium salicylate, potassium salt of methylbenzotriazole, 2-mercaptobenzimidazole, 2,4-dihydroxybenzophenone, 2,6-di-t-butyl-p-cresol, butylated hydroxyanisole, 2,6-di-t-butyl-4-methylphenol, 2,6-di-t-butyl-4-ethylphenol, stearyl-xcex2-(3,5-di-t-butyl-4-hydroxyphenyl)propionate, propyl 3,4,5-trihydroxybenzoate, hydroquinone, and catechol;
bisphenolic antioxidants such as 2,2xe2x80x2-methylenebis(4-methyl-6-t-butylphenol), 2,2xe2x80x2-methylenebis(4-ethyl-6-t-butylphenol), 4,4xe2x80x2-thiobis(3-methyl-6-t-butylphenol), 4,4xe2x80x2-butylidenebis(3-methyl-6-t-butylphenol), 3,9-bis[1,1-dimethyl-2-[xcex2-(3-t-butyl-4-hydroxy-5-methylphenyl)propionyloxy]ethyl]2,4,8,10-tetraoxaspyrro[5,5]undecane, and 4,4xe2x80x2-(2,3-dimethyl-tetramethylene)dipyrrocatechol;
high-molecular phenolic antioxidants such as 1,1,3-tris(2-methyl-4-hydroxy-5-t-butylphenyl)butane, 1,3,5-trimethyl-2,4,6-tris(3,5-di-t-butyl-4-hydroxybenzyl)benzene, tetrakis-[methylene-3-(3xe2x80x2,5xe2x80x2-di-t-butyl-4xe2x80x2-hydroxyphenyl)propionate]methane, bis[3,3xe2x80x2-bis-(4xe2x80x2-hydroxy-3xe2x80x2-t-butylphenyl)butyric acid]glycol ester, 1,3,5-tris(3xe2x80x2,5xe2x80x2-di-t-butyl-4xe2x80x2-hydroxybenzyl)-s-triazine-2,4,6-(1H,3H,5H)trione, and tocopherol;
sulfur-containing antioxidants such as dilauryl 3,3xe2x80x2-thiodipropionate, dimyristyl 3,3xe2x80x2-thiodipropionate, distearyl 3,3xe2x80x2-thiodipropionate, 2-mercaptobenzimidazole, tetrakismethylene-3-(laurylthio)propionatemethane, and stearylthiopropylamide;
phosphorus-containing antioxidants such as triphenyl phosphite, diphenylisodecyl phosphite, phenyldiisodecyl phosphite, 4,4xe2x80x2-butylidene-bis(3-methyl-6-t-butylphenylditridecyl)phosphite, cyclic neopentanetetraylbis(octadecyl)phosphite, tris(nonylphenyl)phosphite, tris(mono- and/or dinonylphenyl)phosphite, diisodecylpentaerythritol diphosphite, 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide, 10-(3,5-di-t-butyl-4-hydroxybenzyl)-9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide, 10-decyloxy-9,10-dihydro-9-oxa-10-phosphaphenanthrene, tris(2,4-di-t-butylphenyl)phosphite, cyclic neopentanetetraylbis(2,4-di-t-butylphenyl)phosphite, cyclic neopentanetetraylbis(2,6-di-t-butyl-4-methylphenyl)phosphite, 2,2-methylenebis(4,6-di-t-butylphenyl)octyl phosphite, distearylpentaerythritol diphophite, di(2,4-di-t-butylphenyl)phophite, and tetrakis(2,4-di-t-butylphenyl)-4,4-biphenylene phophonite;
alcoholic antioxidants such as erysorbic acid, sodium erysorbate, and isopropyl citrate;
amine antioxidants such as methylated diphenylamine, ethylated diphenylamine, butylated diphenylamine, octylated diphenylamine, laurylated diphenylamine, N,Nxe2x80x2-di-sec-butyl-p-phenylenediamine, and N,Nxe2x80x2-diphenyl-p-phenylenediamine;
hindered-amine antioxidants such as 4-benzoyloxy-2,2,6,6-tetramethylpiperidine, bis(2,2,6,6-tetramethyl-4-piperidinyl)sebacate, bis(1-octyloxy-2,2,6,6-tetramethyl-4-piperidinyl)sebacate, bis(1,2,2,6,6-pentamethyl-4-piperidinyl)sebacate, dimethyl succinate-1-(2-hydroxyethyl)4-hydroxy-2,2,6,6-tetramethylpiperidine and condensed products thereof, and 8-acetyl-3-dodecyl-7,7,9,9-tetramethyl-1,3,8-triazaspyrro[4,5]decane-2,4-dione. These may be used alone respectively or in combinations with each other.
Among the above-exemplified antioxidants, particularly, compounds having a phenolic hydroxyl group are preferably used in the present invention, of which the specific examples include the phenolic antioxidants, the bisphenolic antioxidants, and the high-molecular phenolic antioxidants. If the compound having a phenolic hydroxyl group is used as the antioxidant, there are advantages of not increasing the ash content resultant from strong heating of the vinylpyrrolidone polymer.
As to the antioxidant, particularly, compounds of the below-mentioned general formulae (1) and/or (2) are favorably used besides the above-exemplified antioxidants in the present invention, because the deterioration of the vinylpyrrolidone polymer is, for example, considered to occur due to participation of radical species that form by cleavage of peroxides that are generated by influence of ultraviolet rays and so on, but because the radical scavengers of the below-mentioned general formulae (1) and/or (2) make resonant structures in a radicalized state, and therefore have so high radical scavengeability as to be extremely excellent in the ability to prevent the polymer from deteriorating with the passage of time. 
wherein each of R1, R2, R3, R4, and R5 is, independently of each other, at least one member selected from the group consisting of a hydrogen atom, alkyl groups, substituted alkyl groups, aryl groups, and substituted aryl groups, wherein R1 and R2, or R3 and R4, may be bonded to each other to form a cyclic structure.
Specific examples of the compound of general formula (1) above include thioformamide, thioacetamide, thiopropionamide, N-methylthioformamide, N-methylthioacetamide, and N-ethylthioformamide, and specific examples of the compound of general formula (2) above include thiourea, N,Nxe2x80x2-dimethylthiourea, N,Nxe2x80x2-diethylthiourea, N,Nxe2x80x2-dibutylthiourea, N,Nxe2x80x2-diphenylthiourea, ethylenethiourea, and propylenethiourea, but there is no limiation thereto. Among them, particularly, cases where R4 and/or R5 in formula (2) is a hydrogen atom, in other words, cases where the compound of this formula is thiourea or a derivative therefrom, are preferable because, in such cases, the compound can make four kinds of resonant stabilized structures in a radicalized state and therefore has the highest radical scavengeability. When these compounds of general formulae (1) and/or (2) are used, the amount thereof is preferably in the range of 0.00001 to 10 weight %, more preferably 0.005 to 1 weight %, of the vinylpyrrolidone polymer.
In the present invention, moreover, it is preferable that the radical scavenger is jointly used with a peroxide decomposer. The joint use of the radical scavenger with the peroxide decomposer makes synergistic effects upon the prevention of the deterioration of the vinylpyrrolidone polymer, and therefore can maintain the effects for a longer time or at higher temperature than cases where they are used alone respectively.
The above peroxide decomposer reduces either undecomposed residue of peroxides as used for producing the vinylpyrrolidone polymer, or peroxides which form due to factors such as oxidation of radical species while the vinylpyrrolidone polymer is, for example, preserved or transported. Specific examples of the peroxide decomposer include those which are previously enumerated as the specific examples of the sulfur- or phosphorus-containing antioxidant among the above-exemplified antioxidants. When these peroxide decomposers are used, the amount thereof is preferably in the range of 0.00001 to 10 weight %, more preferably 0.005 to 1 weight %, of the vinylpyrrolidone polymer.
The aforementioned radical scavenger scavenges radicals, which are generated by influence of light, heat, and so on while the vinylpyrrolidone polymer is, for example, preserved or transported, with the result that the radicals are prevented from being chained to frameworks of the polymer. Specific examples of the radical scavenger include, among the above-exemplified antioxidants, those which are previously enumerated as the specific examples of the phenolic antioxidant, the bisphenolic antioxidant, the high-molecular phenolic antioxidant, the alcoholic antioxidant, the amine antioxidant, and the hindered-amine antioxidant, and further, those which are previously enumerated as the specific examples of the compound of general formula (1) and/or (2). In the present invention, among these, particularly, the phenolic antioxidants, the bisphenolic antioxidants, the amine antioxidants, the hindered-amine antioxidants, and thiourea and derivatives therefrom are preferable, and thiourea and derivatives therefrom are more preferable. When these radical scavengers are used, the amount thereof is preferably in the range of 0.00001 to 10 weight %, more preferably 0.005 to 1 weight %, of the vinylpyrrolidone polymer.
In the present invention, it is preferable that the vinylpyrrolidone polymer further contains a radical formation inhibitor, because mixing of this radical formation inhibitor might make more synergistic effects upon the prevention of the deterioration of the vinylpyrrolidone polymer. Specific examples of the radical formation inhibitor include conventional ultraviolet absorbent, metal-deactivating agents, and quenchers. When these radical formation inhibitors are used, the amount thereof is preferably in the range of 0.00001 to 10 weight % of the vinylpyrrolidone polymer.
In the present invention, the method for mixing the vinylpyrrolidone polymer with the antioxidant (including the radical scavenger and the peroxide decomposer) and further, as the need arises, with the radical formation inhibitor is not especially limited, but, in a preferable method, these additives are added to a reaction liquid resultant from the polymerization reaction, or to a solution or dispersion of the vinylpyrrolidone polymer which is obtained by dissolving or dispersing a powder of the vinylpyrrolidone polymer into a solvent.
When the vinylpyrrolidone polymer according to the present invention is formed into an aqueous solution with a concentration of 5 weight %, its pH is preferably in the range of 4 to 12, more preferably 5 to 9. In these pH regions, the stabilization effect due to the antioxidant in the present invention can be exhibited still more effectively. In the case where pH is on the more acidic side than 4, the oxidation deterioration easily occurs, therefore the durability of the aforementioned stabilization effect tends to be damaged.
According to the present invention, the storage stability of the vinylpyrrolidone polymer can surely be judged by the following process. The vinylpyrrolidone polymer or a composition, which is prepared by mixing the vinylpyrrolidone polymer with a storage-stabilizing agent such as antioxidant, is subjected to a forcible test in which the vinylpyrrolidone polymer or the composition is kept heated at 120xc2x0 C. under normal pressure in air for 2 hours. Before and after this forcible test, the vinylpyrrolidone polymer is dissolved into any solvent, in which the vinylpyrrolidone polymer is soluble, in a concentration of not higher than 10 weight %. The respective viscosities of the resultant solutions are measured at 25xc2x0 C. with a capillary viscometer. Respective K values are determined from the resultant measured values, wherein the K values are defined by Fikentscher""s equation. Incidentally, in the case where the judgment of the storage stability of the vinylpyrrolidone polymer is carried out in a solution state, the vinylpyrrolidone polymer is subjected to a promotion test in which the vinylpyrrolidone polymer is kept heated at 50xc2x0 C. in a solution state for 14 days, and then the solution is diluted with any solvent to decrease the content of the vinylpyrrolidone polymer to not higher than 10 weight %, and then the viscosity of the resultant solution is measured to determine the K value. Then, either the reduction ratio of the K value after the forcible test to the initial value of the K value before the forcible test or the reduction ratio of the K value after the promotion test to the initial value of the K value before the promotion test is evaluated. As a result, the storage stability of the vinylpyrrolidone polymer can easily be judged. As to the vinylpyrrolidone polymer (including the composition as prepared by mixing the vinylpyrrolidone polymer with a storage-stabilizing agent such as antioxidant) which has excellent storage stability, the above reduction ratio is suppressed within 5%.
Herein, the K value is a value which is calculated from the measured viscosity and the following Fikentscher""s equation:
(log xcex7rel)/C=[(75K02)/(1+1.5K0C)]+K0
K=1000K0 
wherein: C denotes the grams of the vinylpyrrolidone polymer in 100 ml of solution; and xcex7rel denotes the viscosity of the solution to the solvent.
As to the storage-stabilized vinylpyrrolidone polymer according to the present invention (i.e. the vinylpyrrolidone polymer which is adjusted such that the reduction ratio of the K value defined by Fikentscher""s equation will be within 5% when the vinylpyrrolidone polymer is subjected to a forcible test in which the vinylpyrrolidone polymer is kept heated at 120xc2x0 C. under normal pressure in air for 2 hours or to a promotion test in which the vinylpyrrolidone polymer is kept heated at 50xc2x0 C. in a solution state for 14 days), its storage stabilization process by other than mixing with the aforementioned antioxidant is also adoptable providing that it involves mixing with the storage-stabilizing agent or storage stabilization process by which the above reduction ratio of the K value can be maintained.
In the present invention, as the need arises, the vinylpyrrolidone polymer may be allowed to contain, for example, various additives such as processing stabilizers, plasticizers, dispersants, fillers, aging inhibitors, pigments, and curing agents.
In the preservation process of the present invention, it is important to suppress the oxygen concentration to not higher than 50,000 ppm in a gas phase that contacts with the vinylpyrrolidone polymer. The oxygen concentration is preferably suppressed to not higher than 10,000 ppm, more preferably not higher than 1,000 ppm. The lower the oxygen concentration is, the more effectively the preservation stability of the vinylpyrrolidone polymer can be enhanced. In the case where the oxygen concentration is higher than 50,000 ppm, the physical properties such as molecular weight (K value) of the vinylpyrrolidone polymer change with the passage of time when the vinylpyrrolidone polymer is preserved for a long time or at high temperature. When applying the preservation process of the present invention, product form of the vinylpyrrolidone polymer is not especially limited, but may be any form, for example, lumps, fibers, powders, solutions, films.
Herein, the gas phase is a portion other than a solid phase portion and/or a liquid phase portion as occupied by the vinylpyrrolidone polymer. The oxygen concentration in the gas phase means the absolute amount of oxygen as contained in a gas phase of a unit volume. In the case where the vinylpyrrolidone polymer is put in an airtight container, the oxygen concentration in the gas phase is a ratio of a volume as occupied by oxygen of the gas phase portion in the container under normal pressure to a volume of the gas phase portion. Incidentally, the oxygen concentration, for example, can easily be measured with commercially available oxygen analyzers such as galvanic cell diffusion types or zirconia sensor types.
A preferable example of a first means for suppressing the oxygen concentration in a gas phase that contacts with the vinylpyrrolidone polymer is a process comprising the steps of putting the vinylpyrrolidone polymer into an airtight container and evacuating the container in the preservation process of the present invention.
The specific method for evacuating the container is not especially limited, and the container may be evacuated in conventional manners. In addition, the vacuum degree is not especially limited, either, but it is preferable to achieve the vacuum such that the container may not be broken.
A preferable example of a second means for suppressing the oxygen concentration in a gas phase that contacts with the vinylpyrrolidone polymer is a process comprising the steps of putting the vinylpyrrolidone polymer into an airtight container and displacing air in the container with an inert gas and/or carbonic acid gas in the preservation process of the present invention. Particularly, it is more preferable that the displacement of air in the container (in which the vinylpyrrolidone polymer is put) is carried out with carbonic acid gas, because the preservation stability of the vinylpyrrolidone polymer is enhanced, and because the content of unreacted vinylpyrrolidone remaining in the vinylpyrrolidone polymer can be reduced by hydrolysis during the preservation. Incidentally, it is preferable that the displacement with an inert gas and/or carbonic acid gas is carried out not only after the vinylpyrrolidone polymer has been put into the container, but also in the step of transferring the vinylpyrrolidone polymer from a reservoir therefor to the airtight container. For example, it is preferable to keep conditions such as the reservoir or a transferring line in a state filled with the aforementioned gas.
The aforementioned inert gas is a gas that is poor in the reactivity, and specific examples thereof include: gases on group 0 in the periodic table, such as helium, neon, and argon; and nitrogen gas. Among these, nitrogen gas is preferable because it is easily available.
The specific method to displace air in the container with an inert gas and/or carbonic acid gas is not especially limited, but it is preferable that the volume of the gas, as introduced into the container in the displacement step with the gas, is at least 5 times as large as the capacity of the container. In addition, it is preferable that the introduction of the gas into the container is carried out after removing air by once evacuating the container, because the displacement with the gas can thereby more efficiently be carried out. Furthermore, in the case where the vinylpyrrolidone polymer is a solution, it is preferable to allow the solution to bubble by using a tool, such as a gas-introducing tube, in order to also remove oxygen as dissolved in the solution.
A preferable example of a third means for suppressing the oxygen concentration in a gas phase that contacts with the vinylpyrrolidone polymer is a process comprising the steps of putting the vinylpyrrolidone polymer into an airtight container and enclosing at least one deoxidizing agent together with the vinylpyrrolidone polymer in the container in the preservation process of the present invention.
The deoxidizing agent is not especially limited if it can remove oxygen by a chemical reaction. However, in view of the safety or the easy use, for example, those which are obtained by sealing an inorganic deoxidizing agent such as an iron powder, iron oxide, iron hydroxide, or an organic deoxidizing agent such as L-ascorbic acid, or any mixture thereof in with gas-permeable films are preferable. A moisture keeping agent to facilitate deoxidizing can be sealed together. Specifically, commercially available deoxidizing agents, such as xe2x80x9cAgelessxe2x80x9d made by Mitsubishi Gas Chemical Co., Inc., xe2x80x9cBitaronxe2x80x9d made by Toagosei Co., Ltd., and xe2x80x9cModulanxe2x80x9d made by Nippon Kayaku Co., Ltd., may be used.
In the aforementioned respective means for suppressing the oxygen concentration, the container to put the vinylpyrrolidone polymer into is not especially limited by the material or shape of the container if it can achieve the airtightness. However, containers which are so high airtight as to involve little gas absorption or leakage are preferable. Specifically, examples of the material of the container include glass, metals, and various plastics, and examples of the shape of the container include bottles, cans, and bags.
In the preservation process of the present invention, the first, second, and third means, which are aforementioned as the process for suppressing the oxygen concentration in a gas phase that contacts with the vinylpyrrolidone polymer, are used alone respectively or, preferably, in combinations with each other, whereby the oxygen concentration can effectively be suppressed.
It is preferable that the preservation process according to the present invention is applied also to the steps ranging from after the production of the vinylpyrrolidone polymer to the transfer of the vinylpyrrolidone polymer into a preservation container. For example, it is preferable to suppress the oxygen concentration to not higher than the aforementioned certain concentration in the gas phase in places such as reaction apparatuses, powdering apparatuses, drying apparatuses, reservoirs, and transferring lines just after the production.
The preservation stability of the vinylpyrrolidone polymer preserved by the preservation process of the present invention, for example, can be judged from the reduction ratio between K values before and after a forcible test as measured by the following process. The vinylpyrrolidone polymer, which is preserved in an airtight container, is subjected to a forcible test in which the vinylpyrrolidone polymer is kept heated together with the container at 120xc2x0 C. for 2 hours. Before and after this forcible test, the vinylpyrrolidone polymer is dissolved into any solvent, in which the vinylpyrrolidone polymer is soluble, in a concentration of not higher than 10 weight %. The respective viscosities of the resultant solutions are measured at 25xc2x0 C. with a capillary viscometer. Respective K values are determined from the resultant measured values, wherein the K values are defined by Fikentscher""s equation. The reduction ratio of the K value after the forcible test to the initial value of the K value before the forcible test is evaluated. As a result, the preservation stability of the vinylpyrrolidone polymer can easily be judged. If the resultant reduction ratio is within 5%, the vinylpyrrolidone polymer can stably be preserved without any change of the physical properties even for a long time or at high temperature.
The initial value of the K value of the vinylpyrrolidone polymer, to which the present invention is applicable, is not especially limited, but the present invention is preferably applied to a vinylpyrrolidone polymer having a K value in the range of 15xcx9c150, preferably 15xcx9c120. In addition, because particularly the stability of a vinylpyrrolidone polymer having a high K value is inferior, the present invention is more preferably applied to a vinylpyrrolidone polymer having a K value in the range of 60xcx9c150, preferably 60xcx9c120.
The storage-stabilized vinylpyrrolidone polymer and preserved vinylpyrrolidone polymer according to the present invention are usable for vinylpyrrolidone-polymer-containing compositions that are used in various fields such as medicines, cosmetics, pressure sensitive adhesives or adhesives, paints, dispersants, inks, and electronic parts. In addition, such a vinylpyrrolidone polymer composition, according to the present invention, means not only a composition containing the present invention vinylpyrrolidone polymer which is beforehand mixed with the storage-stabilizing agent such as antioxidant or of which the K value is beforehand adjusted, but also a composition which is not beforehand mixed with the storage-stabilizing agent, but to which there is separately added a material (such as antioxidant) that contributes to the storage stabilization of the vinylpyrrolidone polymer as things turn out.
(Effects and Advantages of the Invention)
The present invention can provide a vinylpyrrolidone polymer, wherein the vinylpyrrolidone polymer has the K value that is prevented from reducing, therefore this vinylpyrrolidone polymer exhibits good heat resistance and storage stability.
The present invention can further provide a composition which contains the vinylpyrrolidone polymer and therefore exhibits good heat resistance and storage stability.
The present invention can further provide a stabilization process by which the heat resistance and the storage stability of the vinylpyrrolidone polymer are enhanced.
By the present invention, even when the vinylpyrrolidone polymer is preserved for a long time or at high temperature, the physical properties such as molecular weight (K value) of the vinylpyrrolidone polymer can be prevented from changing, therefore the vinylpyrrolidone polymer can stably be preserved. In addition, in the case where air in a container in which the vinylpyrrolidone polymer is put is displaced with carbonic acid gas, the content of vinylpyrrolidone which remains as a monomer in the vinylpyrrolidone polymer can be reduced by hydrolysis during the preservation.
Hereinafter, the present invention is more specifically illustrated by the following examples of some preferred embodiments in comparison with comparative examples not according to the invention. However, the present invention is not limited to the below-mentioned examples.
The K values in the examples and the comparative examples were calculated in the following way: the viscosity was measured by the aforementioned viscosity measurement process, that is, a process in which the relative viscosity of a 0.1xcx9c2 weight % aqueous solution of each vinylpyrrolidone polymer of the examples and the comparative examples was measured at 25xc2x0 C. with a capillary viscometer; and the resultant measured viscosity was applied to Fikentscher""s equation as aforementioned.